Osteoporosis is the primary public health threat for the aging population. Osteoporosis has been likened to obesity of the bone because normal bone mass is lost as it is replaced with adipose tissue. A well recognized risk factor for development of osteoporosis is the decline of estrogen secretion in women at menopause. A newly recognized risk factor is a high fat diet and obesity. An underappreciated aspect of this bone health crisis is the contribution of exposure to environmental obesogens, contaminants that disrupt the homeostatic controls of adipogenesis and energy balance. A growing number of environmental contaminants, including organotins and phthalates, are being recognized for their ability to activate peroxisome proliferator activated receptor gamma (PPAR?), the master regulator of adipocyte differentiation. Activation of PPAR? in bone is associated with increased adipogenesis and decreased osteogenesis. Since PPAR? is poised at the apex of a regulatory network that controls multi-potent marrow stem cell (MSC) differentiation, it is posited that environmental obesogens are bone marrow toxicants. What is unclear is how exposure to both a high fat diet and environmental obesogens cooperate to modify bone homeostasis. The long term goal is to determine how activation of nuclear receptors in the bone marrow by environmental contaminants modifies MSC differentiation and how skewed MSC differentiation impacts bone marrow function. The objective of this proposal is to examine how exposure to a high fat diet and environmental obesogens cooperate to impair osteogenesis. It is hypothesized 1) that environmental obesogens induce adipogenesis and suppress osteogenesis through activation of PPAR? and RXR, which is a central point of regulatory control in directing MSC differentiation between osteogenic and adipogenic lineages, 2) that co-exposure to dietary fatty acids facilitates adipogenesis and 3) that exposure to a high fat diet will synergize with environmental obesogens to accelerate the progression of osteo- porosis in vivo. By pursuing the following two Specific Aims these hypotheses will be tested: 1) Determine the functional interactions of fatty acid and obesogen exposure and their effects on the mechanisms that control MSC differentiation. Alterations in central transcriptional mechanisms that control the balance between adipogenic and osteogenic differentiation that are induced by a phthalate, an organotin, and dietary fatty acids in primary MSC cultures and define the qualitative/quantitative effects of co-exposure will be delineated. 2) Examine the effect of a high fat diet on obesogen-induced osteoporosis in vivo. The effect of low dose tributyltin (TBT) exposure and diets differing in fat content in intact and ovariectomized female C57BL/6 mice on bone structure, quality and expression of mediators of adipogenesis, osteogenesis and osteoclast activity will be investigated. Given the growing aging population that is already at risk for development of osteoporosis, it is urgent that the molecular mechanisms driving diet- and contaminant-driven suppression of osteogenesis be identified so that appropriate approaches can be developed to prevent the onset/progression of disease. PUBLIC HEALTH RELEVANCE: Osteoporosis is the primary public health threat for the aging population, particularly for post-menopausal women. Exposures to a high fat diet and environmental obesogens, specifically contaminants that activate the master regulator of fat formation, are underappreciated contributors to the risk of developing osteoporosis. Studies in this proposal are designed to define the molecular mechanisms by which phthalates and organotins cause fat formation in bone marrow and suppress bone formation and to define how these processes are exacerbated by consumption of a high fat diet.